The technology pathway to sustainable aviation

In startups we trust

Photo credit: KLM Royal Dutch Airlines

Aviation accounts for 2.5% of global CO2 emissions and may contribute up to 3.5% of warming. The sector's share of emissions will rise in coming years. 

Flygskam is a Swedish word meaning "flight shame". Climate activist Greta Thunberg made it popular. To flygskam is to discourage people from flying. Asking them to transition to less-polluting transport methods or simply to stay home.

The term implies a binary option—stop flying and we live, or keep flying and we die. This ultimatum ignores aviation's economic and human contributions. Aviation directly contributes 2.5% of US GDP and indirectly 5+%.

More importantly, civil aviation contributes to global peace. Connection encourages empathy.

At the formation of the International Civil Aviation Organization (ICAO) in 1943, President Roosevelt encouraged equal international access to aviation. Roosevelt said, "I hope you will not dally with the thought of creating great blocs of closed air, thereby tracing in the sky the conditions of future wars." Here, FDR is referring to empires of the past who attempted to dominate by control of the sea. Their attempts to restrict passage almost always resulted in conflict. Open skies offer a pathway to peace. Closed skies, like the closed sea cordons of the past, a track to conflict. 

The answer is not to limit flying. Instead, we must innovate to move forward.

Commercial aviation suffers from a persistent lack of innovation. The duopoly shared by Boeing and Airbus stymies innovation. Startups, who must live or die by the products they ship, offer hope. It is in startups, therefore, we must trust (and invest). 

Airlines, for their part, should support a dynamic approach. Open-mindedness to integrating and deploying cutting-edge technologies is key. Here, it also makes sense to build the muscles capable of operationalizing cutting-edge technology. Corporate venture arms, like JetBlue Tech Ventures or United Airlines Ventures, allow airlines to study emerging technologies in their infancy. Regulations and customer pressure help push the adoption of those technologies at scale.

Aircraft propulsion, fuel sources, aerodynamics and materials all offer room for greater efficiency.  Engines and fuel sources offer the most direct benefit and near-term potential. The technologies most promising include: Sustainable Aviation Fuels (SAFs), electric aircraft, and hydrogen (H2). Each has its strengths and limitations, but all must be part of the solution.

Sustainable Aviation Fuels (SAF) are “drop-in” technologies, meaning they are a near-equal substitute for existing Jet A/A-1 fuel. SAFs are greener than Jet A because they are renewably sourced from feedstock or carbon sequestration, which offsets the carbon released when they are burned.

Municipal waste, agricultural residues and waste lipids are the basis for bio-SAF. Each time we introduce fossil jet fuel, we create a new source of carbon emissions. By contrast, bio-SAFs use input materials that degrade back into the atmosphere anyway. In this way, bio-SAFs do not introduce new carbon emissions. Bio-SAFs, though, are not as green as we'd like. Current bio-SAF conversion methods (like HEFA) only yield 70-80% GHG savings.

Power-to-Liquids (PtL) is a better alternative. PtL sequesters carbon from the air and uses clean electricity to refine it. Under this process, GHG emissions are lowered by 99%. The product, electro-SAF, is also scalable since atmospheric carbon is readily available.

Airplanes have long lifespans and the scalability of SAFs dramatically improve emissions on aircraft that will continue to fly for a long time to come. Besides, Jet A and SAFs have the highest power-density ratio to volume of the viable options. Electrics and hydrogen cannot offer the same range, speed or economic performance. Even if we phase them out in the future, the industry must lean into SAFs now.

But the challenges facing SAFs today are multi-fold.

First, SAF supply won't be as green as we'd like until electro-SAFs are manufactured at scale.

Second, SAF supply is small - less than 0.1% of jet fuel consumed in 2020. Scaling too, is taking longer than expected. Supply chains have come up short because of lack of investment. This of course is caused by limited demand. Which is due to high prices. It’s the classic chicken and egg conundrum associated with maturing new technologies. Fortunately, low-priced PtL production is on its way. Commercial electro-SAFs are set to ship next year, but it'll take time to develop and scale the solution.

Third, economic incentives to scale SAF production haven’t been aligned. Or at least, they haven’t been as aligned as they could be. Bio-SAFs currently cost 3 times as much as fossil fuel. McKinsey forecasts that the cost of SAFs will come down, but only to twice the price of fossil fuel by 2050.

If that is true, the market for air travel could contract.

But not everyone agrees that SAF production will need to be that expensive. Prometheus, a startup producing electro-SAF, recently signed American Airlines as a customer. The price for 10M gallons of jet fuel? One cent less than fossil jet fuel. Whether that price is sustainable long term remains to be seen. 

If electro-SAF prices are actually lower than Jet A, SAFs can stimulate demand. If not, the air travel market could contract because of higher airfares. A doubling of jet fuel prices would add an estimated 20% to fares. At a conservative -1 price elasticity, that would mean 20% fewer passengers.

The key is to create an economic incentive structure that encourages adoption of SAF. Carbon pricing, public infrastructure investments and public pressure offer an effective cocktail. 

Carbon pricing schemes would tax fossil jet fuel. SAFs could then be offered at parity or less than fossil jet fuel. This would in turn, drive up demand for SAFs. In the short run, that demand spike could cause SAF prices to rise (on limited supply). But it would also speed up production ramps and encourage other makers to join the race. 

Governments in the US and Europe are already bolstering their support for SAF. In May, US lawmakers introduced legislation creating a tax credit for SAF blenders. Europe’s mandate to cut emissions 55% by 2030 (vs 1990 levels) is also encouraging demand for SAFs.  In addition, governments should invest in SAF R&D and infrastructure. Research grants help mature the technologies necessary to scale. Infrastructure subsidies accelerate the scaling of supply.

Passengers, too, can incentivize airlines to speed up their transition to SAFs. The first airlines to adopt SAFs will experience a brand halo and modest premiums to revenue. Today the greenest airlines have the newest airplanes and highest average seat densities.

Frontier claims to be America's greenest airline. It packs 186 seats on an Airbus A320. United flies just 150 seats on the same airplane. The onboard experience, too, is very different. Waiting for behavioral change, though, is the hardest part to solving any structural problem. Switching airlines, particularly for frequent fliers, is a lot to ask. SAFs, however, allow passengers to reduce their carbon footprint without compromising experience.

Electric aircraft are a promising option for short-haul regional travel (sub 500NMi). Electric aircraft promise simplified maintenance protocols and increased reliability. Electricity itself, too, is a less expensive propulsion source than Jet A. Because of this, electric aircraft should have much lower operating costs than regional jets or turboprops. 

If powered by renewable electricity, electric aircraft have zero operating emissions. Electric engines do not emit greenhouse gases or other noxious tailpipe emissions. Renewable energy production isn’t a given, though. In 2010 only 10% of US grid energy came from renewables. NREL estimates the number could increase to 80% by 2050. The electric airplane is only as green as the energy that charges it. 

If scaled, electric regional aircraft could serve many use cases. Network carriers could use electric aircraft to replace and supplement their existing regional aircraft fleets. Startups could begin connecting regional destinations in a point-to-point format. Regionally, this would enable a time and cost effective alternative to driving.

NASA's white paper on Regional Air Mobility (RAM) illustrates the massive potential of efficient RAM aircraft. Americans live an average of 16 minutes from their nearest airport. There are 5,050 public use airports in the United States. Yet, just 30 airports account for 70% of passenger traffic. The average American adult makes two car trips per day. If 1% of existing American drivers transitioned to the skies on 19-passenger RAM vehicles at 80% load factors, daily flights in the National Airspace System would increase sevenfold (from 45,000 today to 320,000).

Cost is the primary barrier to growing regional aviation today. This is where electric aviation offers breakthroughs. Fuel and maintenance are two of the biggest drivers of cost in aviation. A Beech 1900 burns 110 gallons per hour. At $2.02 per gallon of Jet A (Delta’s 2019 fuel cost), it consumes $222 of fuel per hour. Eviation’s Alice consumes 433 kWhs of energy per hour. At 10.84¢ per KWh, Alice consumes a mere $47 per hour in energy. 

Heart Aerospace estimates maintenance activities to cost 100 times less than on turbine engines. Electric aircraft though have added costs as well. Rather than expensive engine overhauls, current battery systems need to be replaced frequently. Batteries are a massive driver of costs. Batteries also do not shed weight as they discharge. This means aircraft batteries must be up sized for the same mission (vs. turbine aircraft).

The weight and cost of batteries lead Leeham News to conclude electric aircraft are infeasible substitutes. Leeham believes battery packs for aviation are likely to cost $400-$500 per KWh. That is some 3 times more expensive than EV batteries which fell to $137 per KWh in 2020. The delta is explained by certification requirements and limited production volumes.

Certification requirements do make aircraft more expensive than automobiles. Demand for aviation batteries, though, is likely to be large enough to achieve economies of scale. Take, for instance, a manufacturer that delivers 1,400 units per year over a 10 year period. If each aircraft required a battery swap twice per year, the OEM would need to supply 31,000 batteries per annum. That is more than the number of Chevy Bolts delivered last year.

The aviation use case stresses batteries to their limits. Reserve requirements and weight sensitivity mean the battery is sized for its mission. This also means that the battery's capacity is only a small percent depleted when it's swapped. The remaining capacity still has significant residual value for other uses. Grid-storage, buses or trucks are all natural use cases for used aircraft batteries.

Modern aircraft also make use of lighter airframe structures and improved aerodynamics. Both of these make more room for battery weight or payload.

Voilá! The recipe for an electric regional aircraft: battery density and cost improvements, combined with modern materials and aerodynamics.

Hydrogen (H2) is likely to power regional, mid and, eventually, long haul aircraft. Design complexity scales in this way as well.

Hydrogen has many benefits. It is abundant, and has several manufacturing pathways. It also eliminates CO2 emissions in flight, though not H2O or NOx. Hydrogen-powered aircraft reduce total climate effects of aviation by 75-90% (compared to fossil jet). 

Hydrogen's challenges are technology, supply chain, commercial maturity and certification.

While more dense than Jet A on a mass basis, hydrogen is about four times less dense on a volume basis. This means hydrogen requires more space to carry the same specific energy. Hydrogen’s volatile nature usually involves storing it in the fuselage, rather than the wings. This means hydrogen-powered aircraft need bespoke designs to accommodate a large fuel tank in the fuselage. Blended wing bodies offer one such concept. 

Burdened by the tiny nature of the H2 molecule, hydrogen is difficult to transport. It cannot flow over existing oil pipeline infrastructure where leakage is likely. The most logical way to produce H2 is via micro-sites near where it'll be consumed. Those manufacturing sites aren't built yet. To begin building them, the market has to demonstrate commercial acceptance. 

Hydrogen fuel cells have been in development since the mid 2000s. Auto giants continue to bring a limited number of hydrogen-powered cars to market. While the technology seems to work fine, it hasn't caught on yet. Electrics promise simplified maintenance and lower operating costs. Proof of commercial acceptance will have to come from a segment other than automotive.

Beyond H2 supply, there are several other ecosystem elements that need focus. There is no workforce that can service hydrogen fuel cell aircraft. Nor is there widespread manufacturing of hydrogen propulsion components on or off the aircraft.

On certification, there is no current certification basis for hydrogen powered aircraft. The many startups working in this space will work with regulators to establish one. They are simply behind their electric counterparts.

All this means hydrogen is a promising long term option. But it will take longer to mature and scale than industry would like. Startup ZeroAvia plans a 2023 entry-into-service (EIS) date for its first aircraft. Airbus on the other hand, is targeting a 2035 EIS  for its first hydrogen-powered aircraft. The latter seems more likely. 

Hydrogen, it seems, has always been the propulsion source of the future. Perhaps it is, but it's not here yet.

Industry should support these promising technologies. Each has a unique role to play in aviation’s journey to sustainability. SAFs offer dramatic near-term GHG reductions at similar prices to fossil jet. Electric aircraft have the potential to unlock affordable regional transportation by air. For many frequent commutes, doing so would mean a structural shift from the ground to the air. Hydrogen offers a pathway to alternate propulsion for all segments of air travel.

For their part, airlines can help accelerate the pace of maturity for these technologies. Directly, airlines offer operational design advice and bolster investor confidence for early-stage companies. Indirectly, airlines have deep ties to government. These links encourage smart and achievable emissions standards.

Airlines can also foster the adoption of carbon pricing schemes. The aim is to credit efficient aircraft operations against the cost of emissions. Thus, airlines are incentivized to adopt greener aviation tech. JetBlue and Joby Aviation recently announced a pilot of this concept. Others should follow suit. 

Technology allows us to stay connected in previously unimaginable ways. As our distant connections deepen, so too does our empathy for one another. Aviation allows for the existence of a network of super highways stitching civilizations together. 

That fabric is important. Grounding aviation is not the answer. Inaction, though, is just as bad. Industry, regulators and entrepreneurs must nurture technology advancements and innovation. In this space, it's also too early to be discriminatory. No single technology is the clear winner. It’s likely we’ll need all of them to make aviation truly sustainable. For now, industry should support the development of them all. The market will see the weakest be weeded out on their own.